Solid-state imaging device with layered microlenses and method for manufacturing same

ABSTRACT

A solid-state imaging device includes: a first lens layer; and a second lens layer, wherein the second lens layer is formed at least at a periphery of each first microlens formed based on the first lens layer, and the second lens layer present at a central portion of each of the first microlenses is thinner than the second lens layer present at the periphery of the first microlens or no second lens layer is present at the central portion of each of the first microlenses.

CROSS REFERENCES TO RELATED APPLICATIONS

This application is a continuation of and claims the benefit under 35U.S.C. § 120 of U.S. patent application Ser. No. 15/653,819, titled“SOLID-STATE IMAGING DEVICE WITH LAYERED MICROLENSES AND METHOD FORMANUFACTURING SAME,” filed on Jul. 19, 2017, now U.S. Pat. No.10,355,038, which is a continuation of and claims the benefit under 35U.S.C. § 120 of U.S. patent application Ser. No. 14/861,175, titled“SOLID-STATE IMAGING DEVICE WITH LAYERED MICROLENSES AND METHOD FORMANUFACTURING SAME,” filed on Sep. 22, 2015, and now U.S. Pat. No.9,741,757, which is a continuation of and claims the benefit under 35U.S.C. § 120 of U.S. application Ser. No. 13/613,261, titled“SOLID-STATE IMAGING DEVICE WITH LAYERED MICROLENSES AND METHOD FORMANUFACTURING SAME,” filed on Sep. 13, 2012, and now U.S. Pat. No.9,159,760, which claims the benefit under 35 U.S.C. § 119 of JapanesePatent Application No. JP2011-217423 filed on Sep. 30, 2011. Each of theforegoing applications is incorporated herein by reference in itsentirety.

FIELD

The technology of the present disclosure relates to a solid-stateimaging device, a method for manufacturing the solid-state imagingdevice, and an electronic apparatus, and particularly to a solid-stateimaging device including microlenses on photodiodes, a method formanufacturing the solid-state imaging device, and an electronicapparatus.

BACKGROUND

CMOS (complementary metal oxide semiconductor) solid-state imagingdevices are known to be classified into a front-illumination type and arear-illumination type. A solid-state imaging device of either of thetwo types includes a pixel region in which a plurality of unit pixelsare formed on a semiconductor base substrate and each of the unit pixelsis formed of a photodiode that works as a photoelectric converter and aplurality of pixel transistors.

In a front-illumination solid-state imaging device, the front surface ofa substrate on which a multilayer wiring layer is formed works as alight receiving surface, and light is incident on the front surface ofthe substrate.

In a rear-illumination solid-state imaging device, the rear surface ofthe substrate that faces away from the front surface of the substrate onwhich a multilayer wiring layer and pixel transistors are formed worksas a light receiving surface, and light is incident on the rear surfaceof the substrate.

The photodiodes are isolated from each other by a device isolatingregion formed of an impurity diffusion layer. Further, the multilayerwiring layer, in which a plurality of wiring lines are disposed, isformed via an interlayer insulating layer on the front surface of thesemiconductor base substrate, on which the pixel transistors are formed.

In a front-illumination solid-state imaging device, the wiring lines areformed in regions other than the photodiodes. On-chip color filters andmicrolenses are sequentially formed via a planarization layer on themultilayer wiring layer. The on-chip filters are formed, for example, ofan array of red (R), green (G), and blue (B) filters.

In a rear-illumination solid-state imaging device, the wiring lines canbe formed irrespective of the positions of the photodiodes. Aninsulating layer, on-chip color filters, and microlenses aresequentially formed on the rear surface of the semiconductor basesubstrate, which works as the light receiving surface of thephotodiodes.

In a rear-illumination solid-state imaging device, since the multilayerwiring layer does not constrain light from entering the photodiodes inany manner, each of the photodiodes can be provided with a largeopening. Further, the distance from the photodiodes to the microlensescan be shortened, as compared with that in a front-illuminationsolid-state imaging device. Shortening the distance can improve theability of the microlenses to collect light, whereby obliquely incidentlight can also be efficiently introduced. As a result, the sensitivityof the solid-state imaging device can be increased.

To improve the ability of the microlenses to collect light, for example,the curvature of each of the microlenses may be increased, or therefractive index of the material of which the microlenses are made maybe increased (see JP-A-2007-53318, JP-A-1-10666, JP-A-2008-277800, andJP-A-2008-9079).

SUMMARY

The solid-state imaging device described above is typically required toimprove their sensitivity characteristics by optimizing the shape of themicrolenses.

Thus, it is desirable to provide a solid-state imaging device havingexcellent sensitivity characteristics, a method for manufacturing thesolid-state imaging device, and an electronic apparatus using thesolid-state imaging device by using the technology of the presentdisclosure.

An embodiment of the technology of the present disclosure is directed toa solid-state imaging device including a first lens layer and a secondlens layer, and the second lens layer is formed at least at a peripheryof each first microlens formed based on the first lens layer. The secondlens layer present at a central portion of each of the first microlensesis thinner than the second lens layer present at the periphery of thefirst microlens, or no second lens layer is present at the centralportion of each of the first microlenses.

Another embodiment of the technology of the present disclosure isdirected to an electronic apparatus including the solid-state imagingdevice described above and a signal processing circuit that processes anoutput signal from the solid-state imaging device.

Still another embodiment of the technology of the present disclosure isdirected to a method for manufacturing a solid-state imaging deviceincluding forming first microlenses having an inter-pixel gaptherebetween based on a first lens layer, and forming a second lenslayer at least at a periphery of each of the first microlenses. In theformation of the second lens layer, the second lens layer formed at acentral portion of each of the first microlenses is thinner than thesecond lens layer formed at the periphery of the first microlens, or nosecond lens layer is present at the central portion of each of the firstmicrolenses.

According to the solid-state imaging device described above and thesolid-state imaging device manufactured using the manufacturing methoddescribed above, the first microlenses having an inter-pixel gaptherebetween are formed based on the first lens layer, and the secondlens layer is formed at the periphery of each of the first microlenses.The second lens layer formed at the periphery of each of the firstmicrolenses fills the inter-pixel gap between the first microlenses. Thearea of each of the microlenses in a plan view is therefore enlarged,whereby the ability of the microlens to collect light is improved. As aresult, the sensitivity characteristics of the solid-state imagingdevice are improved. An electronic apparatus having excellentsensitivity characteristics can thus be configured by incorporating thesolid-state imaging device.

According to the embodiments of the technology of the presentdisclosure, a solid-state imaging device having excellent sensitivitycharacteristics, a method for manufacturing the solid-state imagingdevice, and an electronic apparatus can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view showing the configuration of a solid-state imagingdevice according to an embodiment;

FIG. 2 is a cross-sectional view showing the configuration of a pixelsection of the solid-state imaging device according to the embodiment;

FIG. 3 is a plan view showing the configuration of a first lens layer inthe solid-state imaging device;

FIGS. 4A to 4D show various configurations of microlenses in thesolid-state imaging device according to the embodiment;

FIGS. 5A to 5C show configurations of variations of the microlenses inthe solid-state imaging device;

FIGS. 6A to 6D are manufacturing step diagrams showing an embodiment ofa method for manufacturing the solid-state imaging device;

FIGS. 7E to 7H are other manufacturing step diagrams showing theembodiment of a method for manufacturing the solid-state imaging device;

FIGS. 8A to 8C show steps of forming a second planarization layer in thesolid-state imaging device;

FIG. 9 describes the step of forming first microlenses in thesolid-state imaging device according to the embodiment;

FIG. 10 describes the configuration of the microlenses in thesolid-state imaging device according to the embodiment;

FIGS. 11A and 11B describe the configuration of the microlenses in thesolid-state imaging device according to the embodiment; and

FIG. 12 shows the configuration of an electronic apparatus.

DETAILED DESCRIPTION

An example of the mode for carrying out the technology of the presentdisclosure will be described below. It is, however, noted that thetechnology of the present disclosure is not limited to the followingexample.

The description will be made in the following order.

1. Embodiment of solid-state imaging device

2. Embodiment of method for manufacturing solid-state imaging device

3. Embodiment of electronic apparatus

<1. Embodiment of Solid-State Imaging Device>

Example of Configuration of Solid-State Imaging Device: SchematicConfiguration Diagram

A specific form of a solid-state imaging device according to the presentembodiment will be described below.

FIG. 1 is a schematic configuration diagram showing a MOS (metal oxidesemiconductor) solid-state imaging device as an example of thesolid-state imaging device.

A solid-state imaging device 10 shown in FIG. 1 is formed of a pixelsection (what is called imaging region) 13 and a peripheral circuitsection. In the pixel section 13, pixels 12, each of which includes aphotodiode, are regularly and two-dimensionally arranged as a pluralityof photoelectric converters on a silicon substrate or any othersemiconductor base substrate. Each of the pixels 12 includes aphotodiode and a plurality of pixel transistors (what is called MOStransistor).

The plurality of pixel transistors can be formed, for example, of threetransistors: a transfer transistor; a rest transistor; and anamplification transistor. The plurality of pixel transistors canalternatively be formed of four transistors: those described above andan additional selection transistor.

The peripheral circuit section is formed of a vertical drive circuit 14,column signal processing circuits 15, a horizontal drive circuit 16, anoutput circuit 17, and a control circuit 18.

The control circuit 18 produces a clock signal and a control signalbased on a vertical sync signal, a horizontal sync signal, and a masterclock, and the produced clock signal and control signal serve asreferences according to which the vertical drive circuit 14, the columnsignal processing circuits 15, the horizontal drive circuit 16, andother components operate. The control circuit 18 inputs the signals intothe vertical drive circuit 14, the column signal processing circuits 15,the horizontal drive circuit 16, and other components.

The vertical drive circuit 14 is formed, for example of a shiftresistor. The vertical drive circuit 14 selects and scans the pixels 12in the pixel section 13 sequentially in the vertical direction on a rowbasis and supplies the column signal processing circuits 15 with pixelsignals based on signal charge produced in accordance with the amount oflight received by the photoelectric conversion devices in the selectedpixels 12 via vertical signal lines 19.

Each of the column signal processing circuits 15 is disposed inaccordance with a certain unit of pixels 12, for example, on a pixelcolumn and performs noise reduction or otherwise processes signalsoutputted from the pixels 12 in a single row by using a signal fromblack reference pixels (formed around effective pixel region) on a pixelcolumn basis. That is, the column signal processing circuits 15 performCDS (correlated double sampling) for removing a fixed pattern noisespecific to the pixels 12, signal amplification, and other types ofsignal processing. Each of the column signal processing circuits 15 hasa horizontal selection switch (not shown) provided at its output stageand connected to a horizontal signal line 11.

The horizontal drive circuit 16 is formed, for example, of a shiftresistor. The horizontal drive circuit 16 sequentially selects each ofthe column signal processing circuits 15 by successively outputting ahorizontal scan pulse and outputs pixel signals from each of the columnsignal processing circuits 15 to the horizontal signal line 11.

The output circuit 17 performs signal processing on the signalssequentially supplied from each of the column signal processing circuits15 through the horizontal signal line 11 and outputs the processedsignals.

When the solid-state imaging device 10 described above is used as arear-illumination solid-state imaging device, no wiring layer is formedon the rear surface (what is called light receiving surface) on the sideon which light is incident, but a wiring layer is formed on the frontsurface facing away from the light receiving surface.

Example of Configuration of Solid-State Imaging Device: Pixel Section

FIG. 2 is a cross-sectional view showing a key portion that forms asingle pixel of the solid-state imaging device according to the presentembodiment.

A solid-state imaging device 20 shown in FIG. 2 includes a plurality ofphotodiodes (PDs) 22 on the side of a semiconductor base substrate 21 onwhich light is incident. Each of the PDs 22 is formed in a unit pixel 29of the solid-state imaging device 20. An insulating layer 23 formed of asingle layer or multiple layers is formed on the semiconductor basesubstrate 21.

In the rear-illumination solid-state imaging device 20 shown in FIG. 2,a circuit section including a multilayer wiring layer and pixeltransistors is formed on the surface facing away from the surface onwhich light is incident, but the configuration of the circuit section isomitted in FIG. 2.

The insulating layer 23, when formed of a single layer, is made, forexample, of SiO. The insulating layer 23, when formed of multiplelayers, is formed of multiple layers having different refractive indicesbased on the configuration of an antireflection layer. For example, theinsulating layer 23 is formed of two layers: a hafnium oxide (HfO₂)layer and a silicon oxide layer.

An inter-pixel light blocking layer 24 is formed on the insulating layer23 described above, specifically, along the boundary between the unitpixels 29 in correspondence with an opening of each of the PDs 22 of thesolid-state imaging device 20. The inter-pixel light blocking layer 24is formed, for example, of a layer made of W, Al, Cu, or any othersuitable metal or carbon black, a titanium black, or any other suitableorganic material. The inter-pixel light blocking layer 24 preventsincident light from leaking to an adjacent pixel and hence suppresscolor mixture in the solid-state imaging device 20.

A first planarization layer 25 made, for example, of an acrylic resin isformed on the insulating layer 23 and the inter-pixel light blockinglayer 24. The first planarization layer 25 planarizes protrusions andrecesses resulting from the inter-pixel light blocking layer 24 andother factors. The first planarization layer 25 further reducesapplication unevenness that occurs when color filters are formed on thesemiconductor base substrate 21, for example, in a spin applicationprocess.

Color filters 26 are formed on the first planarization layer 25. Thecolor filters 26 are formed of a variety of optical filters, such asRED, GREEN, BLUE, YELLOW, CYAN, MAGENTA optical filters. In theformation of the color filters 26, the layer thickness for each of thecolors is so optimized that an optimum color image is outputted. Thesurface of the entire color filters 26 therefore has protrusions andrecesses to some extent.

A second planarization layer 27 for planarizing the protrusions andrecesses resulting from the surfaces of the color filters 26 is formedon the color filters 26. The second planarization layer 27 is made of amaterial that has thermal fluidity and thermal curability and forms acured layer when a thermal treatment is finished, such as an acrylicresin, a styrene resin, and a styrene-acryl copolymerizing resin. Theplanarization ability of the materials described above decreases in thefollowing order: an acrylic resin, a styrene-acryl copolymerizing resin,and a styrene resin. The second planarization layer 27 is thereforepreferably made of an acrylic resin, which excels in the planarizationability.

A buffer layer 28 is formed on the second planarization layer 27. Thebuffer layer 28 is made, for example, of SiO or SiON. The buffer layer28 is formed to prevent corrugation due to the difference in film stressor reduce the reflectance, as will be described later.

Microlenses 30 are formed on the buffer layer 28.

The microlenses 30 are formed of a first lens layer 31 and a second lenslayer 33 formed on the first lens layer 31. The first lens layer 31 isformed over the second planarization layer 27 and forms firstmicrolenses 32. The second lens layer 33 is then so formed that itcovers the first microlenses 32. The second planarization layer 27 isformed to increase uniformity of the first microlenses 32 formed overthe color filters 26.

The first lens layer 31, which forms the microlenses 30 in thesolid-state imaging device 20, is made of one or more of the followingmaterials: a resin, SiN, and SiON.

The resin is, for example, a metal-oxide-containing resin in which metalfine particles are dispersed. Examples of the resin in which metal fineparticles are dispersed include an acrylic resin, a styrene-based resin,a novolac resin, an epoxy-based resin, a polyimide-based resin, and asiloxane-based resin. Examples of the metal fine particles that aredispersed in the resin include a zinc oxide, a zirconium oxide, aniobium oxide, a titanium oxide, and a tin oxide. Dispersing a metaloxide in a resin increases the refractive index of the resin.

When the first lens layer 31 is made of SiN and the second planarizationlayer 27 is made of an acrylic resin, formation of SiN directly on thesecond planarization layer 27 may produce corrugation along theinterface between the two layers in some cases due to stress inducedwhen the SiN layer is formed. Corrugation degrades an image produced bythe solid-state imaging device.

Corrugation results from the difference in membrane stress between theresin that form the second planarization layer 27, such as an acrylicresin, and the inorganic material that forms the first lens layer 31,such as SiN.

The magnitude of the membrane stress induced in the materials describedabove increases in the following order: an acrylic resin (secondplanarization layer 27), SiO, SiON, SiN (first lens layer 31).Corrugation occurs because the difference in membrane stress between thesecond planarization layer 27 (acrylic resin) and the first lens layer31 (SiN) is large as described above. To address the problem, the bufferlayer 28 made of SiO or SiON, in which stress of intermediate magnitudeis induced, is formed between the two layers. A buffering effectprovided by the layer in which stress of intermediate magnitude isinduced suppresses corrugation along the interface between the twolayers.

The magnitude of the membrane stress induced in an acrylic resin, astyrene resin, and a styrene-acryl copolymerization resin is as follows:acrylic resin<styrene-acryl copolymerization resin<styrene resin<SiO. Astyrene resin and a styrene-acryl copolymerization resin are moresimilar to SiN, which forms the first lens layer 31, than an acrylicresin. Corrugation is therefore more unlikely to occur along theinterface when a styrene resin or a styrene-acryl copolymerization resinas a second planarization layer 27, in which a greater amount ofmembrane stress is induced than in an acrylic resin, is used than whenan acrylic resin is used.

Even when the first lens layer 31 is made of a metal-oxide-containingresin or corrugation is unlikely to occur along the interface betweenthe first lens layer 31 and the second planarization layer 27, thebuffer layer 28 described above can be provided as an antireflectionlayer.

Consider now a case where the second planarization layer 27 made of anacrylic resin, the buffer layer 28 made of SiON, and the first lenslayer 31 made of SiN are formed over the color filters 26.

In the configuration described above, the magnitude of the refractiveindices n of the layers is as follows: second planarization layer 27(acrylic resin: n is about 1.5)<buffer layer 28 (SiON: n ranges fromabout 1.6 to 1.8)<first lens layer 31 (SiN: n ranges from about 1.85 to2.0).

When the second planarization layer 27 is alternatively made of astyrene resin (n is about 1.6) or a styrene-acryl copolymerization resin(n ranges from about 1.55 to 1.58) as well, the magnitude of therefractive indices n of the layers is as follows: second planarizationlayer 27<buffer layer 28 (SiON)<first lens layer 31 (SiN).

The relationships of the magnitude of the refractive index describedabove indicate that when the buffer layer 28 is made of SiON, thereflectance decreases because the buffer layer 28 has an intermediaterefractive index between those of the second planarization layer 27 andthe first lens layer 31.

As described above, the buffer layer 28 can function as anantireflection layer by employing a configuration in which the bufferlayer 28 has an intermediate refractive index between those of thesecond planarization layer 27 and the first lens layer 31. For example,even when the buffer layer 28 is made of SiO having a refractive index nof about 1.45, the buffer layer 28 can function as an antireflectionlayer by changing the materials, film formation conditions, and otherfactors of the second planarization layer 27 and the first lens layer 31to adjust the refractive indices thereof.

Even when the first lens layer 31 is not made of SiN but is made of ametal-oxide-containing resin having a refractive index comparable withthat of SiN, the reflectance can be reduced as described above.

When the second planarization layer 27 and the buffer layer 28, thelatter of which is made of SiN or SiON, are formed over the colorfilters 26, the distance from the photodiodes 22 to the microlenses 30in the solid-state imaging device 20 increases. The buffer layer 28,however, only needs to be about 5 nm in thickness to provide thebuffering effect. Further, the second planarization layer 27 can beformed to be thin when a selected acrylic resin has thermal fluidity,thermal curability, and thermal shrinkability. Therefore, even when thesecond planarization layer 27 and the buffer layer 28 are formed, andthe distance from the photodiodes 22 to the microlenses 30 increases,the resultant decrease in the sensitivity of the solid-state imagingdevice does not cause a practical problem.

FIG. 3 is a plan view showing the first lens layer 31 formed on thebuffer layer 28.

The broken lines shown in FIG. 3 represent the unit pixels 29 in thesolid-state imaging device 20. The microlenses 30 formed incorrespondence with the unit pixels 29 are preferably so formed thateach of the microlenses 30 has the same size as that of thecorresponding unit pixel 29 in the plan view. Further, the first lenslayer 31 is so formed that it covers the entire unit pixels 29.

The first microlenses 32 formed based on the first lens layer 31 are soformed that adjacent first microlenses 32 have a gap in at least one ofthe direction parallel to horizontally or vertically adjacent pixels(W1) and the direction parallel to diagonally adjacent pixels (W2), asshown in FIG. 3.

To improve the sensitivity characteristics of the solid-state imagingdevice 20, the gaps between adjacent pixels in the directions W1 and W2described above and the distance from the photodiodes 22 to themicrolenses 30 are preferably minimized. Further, when the microlenses30 are formed in a dry etching process, it is necessary to minimize theetching process period. The reason for this is that dark current in thesolid-state imaging device 20 can be suppressed by minimizing plasmadamage to the semiconductor base substrate 21.

A typical solid-state imaging device of related art has square unitpixels 29, as shown in FIG. 3 described above. In this configuration,even when the first microlenses 32 are formed, for example, with no gapin the direction W1, the gaps in the direction W2, each of which isinherently large, still remain. When the microlenses 30 are formed, forexample, in a dry etching process, it is conceivable that the gaps inthe direction W2 are eliminated by performing the etching in a conditionthat reduces the length of the gaps in the direction W2. When the dryetching is performed in a condition that eliminates the gaps in thedirection W2, however, the dry etching period increases, resulting indegradation in dark current characteristics of the solid-state imagingdevice 20. Further, when the processing period increases, wafer-to-waferetching variation increases. As a result, the cross-sectional shape ofthe microlenses 30 varies, which adversely affects the sensitivitycharacteristics of the solid-state imaging device 20.

In contrast, in the solid-state imaging device 20 according to thepresent embodiment, the microlenses 30 are formed by stacking the firstlens layer 31 and the second lens layer 33.

Now, define the thickness of the second lens layer 33 formed on thefirst lens layer 31 as follows: Let Tt be the thickness of the secondlens layer 33 formed at a central portion of each of the firstmicrolenses 32, and let Tb be the thickness of the second lens layer 33formed at the periphery of the first microlens 32, as shown in FIGS. 4Ato 4D. The thus defined Tt and Tb satisfy the following relationship:0≤Tt≤Tb, which represents a configuration in which the second lens layer33 is present at the periphery of each of the first microlenses 32 andthe second lens layer 33 whose thickness is smaller than that of thesecond lens layer 33 present in the periphery is also present at thecentral portion of the first microlens 32 or no second lens layer 33 ispresent at the central portion of the first microlenses 32.

In the thus configured solid-state imaging device 20, the firstmicrolenses 32 are so formed that adjacent ones have a gap w in at leastone of the direction parallel to horizontally or vertically adjacentpixels (W1) and the direction parallel to diagonally adjacent pixels(W2), as shown in FIG. 3 described above. In the condition that allowsthe gap w to be formed, the dry etching period can be shortened, wherebythe increase in dark current in the solid-state imaging device 20 issuppressed.

Further, the microlenses 30 are so formed that the second lens layer 33enlarges the area of each of the first microlenses 32 in the plan view.As a result, the ability of each of the microlenses 30 to collect lightcan be improved, whereby the sensitivity and shading characteristics ofthe solid-state imaging device 20 can be improved.

The second lens layer 33 is made of at least one of the materialsselected from SiON, SiN, SiO, and SiOC (having refractive index of about1.4). When the second lens layer 33 is made of SiO or SiOC, which has arefractive index lower than those of SiON and SiN, one of which formsthe first lens layer 31, the second lens layer 33 also functions as anantireflection layer on the microlenses 30.

FIGS. 4A to 4D show configurations that satisfy the relationship betweenTt and Tb described above, 0≤Tt≤Tb, in the first lens layer 31 and thesecond lens layer 33, which form the microlenses 30.

Microlenses 30A to 30D shown in FIGS. 4A to 4D correspond to states inwhich second lens layers 33 having different thicknesses are formed onthe first microlenses 32. The microlenses 30A to 30D shown in FIGS. 4Ato 4D are so configured that the first microlenses 32 are formed basedon the first lens layer 31 in a known method of related art and thesecond lens layer 33 is then so formed on the first lens microlenses 32that the thickness of the second lens layer 33 is adjusted in a dryetching process. The method for forming the first lens layer 31 and thesecond lens layer 33 will be described in detail in the description of amethod for manufacturing the solid-state imaging device, which will bedescribed later.

In the microlenses 30A shown in FIG. 4A, the second lens layer 33 isformed over the entire surface of the first lens layer 31. Therelationship between the thickness Tt of the second lens layer 33 at thecentral portion of each of the first microlenses 32 and the thickness Tbof the second lens layer 33 at the periphery of the first microlens 32is Tt<Tb.

Further, the second lens layer 33 is so configured that it is thinnestat the central portion of each of the first microlenses 32 and graduallybecomes thicker with the distance from the central portion toward theperiphery.

In the configuration described above, the second lens layer 33 fills theinter-pixel gap w between the first microlenses 32, whereby theresultant microlenses 30A have no inter-pixel gap w. The area of each ofthe microlenses 30A in a plan view can therefore be enlarged, wherebythe ability of the microlens 30A to collect light can be improved andthe sensitivity characteristics of the solid-state imaging device 20 canbe improved accordingly.

In the microlenses 30B shown in FIG. 4B, the second lens layer 33 isformed over the entire surface of the first lens layer 31 except thecentral portion of each of the first microlenses 32. The configurationcorresponds to a state in which the etching is terminated when Ttbecomes zero in the formation of the second lens layer 33. The thusconfigured microlenses 30B are formed based on the microlenses 30A inthe state shown in FIG. 4A by further etching the second lens layer 33.

The second lens layer 33 is not present at the central portion of eachof the first microlenses 32 but is present at the periphery of the firstmicrolens 32. That is, the second lens layer 33 is formed based on therelationship of 0=Tt<Tb.

In the configuration described above, the second lens layer 33 formed atthe periphery of each of the first microlenses 32 fills the inter-pixelgap w. The resultant microlenses 30B have no inter-pixel gap w. Theconfiguration described above allows the area of each of the microlenses30B in a plan view to be enlarged, whereby the ability of the microlens30B to collect light can be improved and the sensitivity characteristicsof the solid-state imaging device 20 can be improved accordingly.

The microlenses 30C shown in FIG. 4C correspond to a state in which thesecond lens layer 33 is further etched from the microlenses 30B in thestate shown in FIG. 4B.

In the microlenses 30C, the second lens layer 33 is formed over theentire surface of the first lens layer 31 except a central portion ofeach of the first microlenses 32. The central portion of each of thefirst microlenses 32 and therearound is etched along with the secondlens layer 33 by further etching the second lens layer 33. In FIG. 4C,the broken lines 32A each represent the portion of the lens surface ofthe central portion of the first microlens 32 before etched.

The configuration shown in FIG. 4C is achieved by further etching thesecond lens layer 33 from the state in which the central portion of eachof the first microlenses 32 is exposed to a state in which the firstmicrolens 32 exposed through the second lens layer 33 is etched. Asdescribed above, each of the etched regions of the first lens layer 31is enlarged in the direction from the central portion of thecorresponding first microlens 32 toward the periphery thereof by furtheretching the second lens layer 33.

In the configuration of the microlenses 30C shown in FIG. 4C as well,the second lens layer 33 is not present at the central portion of eachof the first microlenses 32 but is present at the periphery of the firstmicrolens 32. That is, the second lens layer 33 is formed based on therelationship 0=Tt<Tb. The second lens layer 33 at the periphery of eachof the first microlenses 32 fills the inter-pixel gap w. The resultantmicrolenses 30C have no inter-pixel gap w.

As shown in FIG. 4C, even in the configuration in which no second lenslayer 33 is formed at the central portion of each of the firstmicrolenses 32 and therearound, the area of each of the microlenses 30Cin a plan view can be enlarged because the second lens layer 33 fillsthe inter-pixel gap w.

Further, even in the configuration in which the central portion of eachof the first microlenses 32 is etched, the second lens layer 33 forms aseries of lens surfaces formed of the first microlenses 32 and thesecond lens layer 33.

The configuration described above allows the ability of each of themicrolenses 30 to collect light to be improved and the sensitivitycharacteristics of the solid-state imaging device 20 to be improvedaccordingly.

The microlenses 30D shown in FIG. 4D correspond to a state in which thefirst lens layer 31 and the second lens layer 33 are further etched fromthe microlenses 30C in the state of shown in FIG. 4C. In FIG. 4D, thebroken lines 32A each represent the portion of the lens surface of thefirst microlens 32 before etched.

In the microlenses 30D, the second lens layer 33 is formed only in thevicinity of the periphery of each of the first microlenses 32 and in theinter-pixel gap w therearound. The etched region of each of the firstmicrolenses 32 is enlarged in the direction from the center toward theperiphery and is larger than the etched region in the configurationshown in FIG. 4C described above. Further, a central portion of each ofthe inter-pixel gaps w of the first lens layer 31 is etched away, as acentral portion of each of the first microlenses 32 is. That is, thesecond lens layer 33 is completely etched away at the central portion ofeach of the inter-pixel gaps w, and the first lens layer 31 exposedthrough the second lens layer 33 at the central portion of theinter-pixel gap w is also etched.

In the configuration of the microlenses 30D shown in FIG. 4D, the secondlens layer 33 is not present at the central portion of each of the firstmicrolenses 32 or the central portion of each of the inter-pixel gaps w.The second lens layer 33 having the thickness Tb is, however, present atthe periphery of each of the first microlenses 32. That is, the secondlens layer 33 is formed based on the relationship 0=Tt<Tb.

Further, the surface formed by etching the first lens layer 31 at thecentral portion of each of the inter-pixel gaps w is connected to thesurface of the second lens layer 33 and the lens surface of the adjacentfirst microlens 32, whereby a series of lens surfaces that form themicrolenses 30D is formed.

As a result, the first microlenses 32, the second lens layer 33 at theperipheries of the first microlenses 32, and the etched surfaces of theinter-pixel gaps w of the first lens layer 31 form the microlenses 30Dwith no inter-pixel gap w.

As described above, even in the configuration in which the first lenslayer 31 formed in the inter-pixel gaps w is also etched, the secondlens layer 33 formed at the periphery of each of the first microlenses32 fills the inter-pixel gap w around the first microlens 32. As aresult, the area of each of the microlenses 30D in a plan view isenlarged, whereby the ability of the microlens 30 to collect light canbe improved and the sensitivity characteristics of the solid-stateimaging device 20 can be improved accordingly.

The configuration of the microlenses 30D shown in FIG. 4D may be furtherso etched that the bottom of the first lens layer 31 in each of theinter-pixel gaps w is penetrated to the second planarization layer 27.In this configuration as well, as long as the second lens layer 33having the thickness Tb is present at the periphery of each of the firstmicrolenses 32, the first microlenses 32, the second lens layer 33, andthe first lens layer 31 in the inter-pixel gaps w form a series of lenssurfaces, whereby the area of each of the microlenses 30D in a plan viewcan be enlarged.

[Variations]

Variations of the microlenses in the solid-state imaging devicedescribed above will next be described. FIGS. 5A to 5C show theconfigurations of microlenses 35 of the variations. The components otherthan the microlenses are the same as those in the embodiment describedabove and will therefore not be illustrated or described.

The microlenses 35 shown in FIGS. 5A to 5C each include a third lenslayer 34 over the first lens layer 31 and the second lens layer 33.

The configuration of microlenses 35A shown in FIG. 5A corresponds to theconfiguration of the microlenses 30A shown in FIG. 4A described above,and the configuration of microlenses 35B shown in FIG. 5B corresponds tothe configuration of the microlenses 30B shown in FIG. 4B describedabove.

In the microlenses 35A shown in FIG. 5A, the third lens layer 34 isformed over the entire surface of the second lens layer 33.

In the microlenses 35B shown in FIG. 5B, the third lens layer 34 is soformed that is covers a central portion of each of the first microlenses32 and the second lens layer 33.

The third lens layer 34 is formed to a substantially uniform thicknessalong the upper surface of the second lens layer 33 both in themicrolenses 35A and the microlenses 35B.

In the microlenses 35C shown in FIG. 5C, the third lens layer 34 isprovided over the first lens layer 31 and the second lens layer 33 andhas a flat surface. The third lens layer 34 may thus have a flatsurface.

Each of the third lens layers 34 shown in FIGS. 5A to 5C, when made of amaterial having a refractive index lower than that of the material thatforms the first lens layer 31, functions as an antireflection layer onthe surface of the microlenses 35.

Table 1 below shows the relationship of the refractive indices of thesecond lens layer 33 and the third lens layer 34 with the refractiveindex of the first lens layer 31 made of SiON or SiN.

TABLE 1 First lens layer Second lens layer Third lens layer (1)Refractive index = c Comparable with c Lower than c (2) Refractive index= c Higher than c Lower than c

In Table 1, it is assumed that the first lens layer 31 has a refractiveindex c, and the second lens layer 33 is made of a material (1) having arefractive index comparable with the refractive index c of the firstlens layer 31 or a material (2) having a refractive index higher thanthe refractive index c of the first lens layer 31. The third lens layer34 is made of a material having a refractive index lower than therefractive index c of the first lens layer 31.

In the configuration shown in the row (1) in Table 1, the third lenslayer 34 functions as a monolayer antireflection layer. To this end, thethird lens layer 34 is made of SiO (refractive index of about 1.45) orSiOC (refractive index of about 1.4).

In the configuration shown in the row (2) in Table 1, the second lenslayer 33 and the third lens layer 34 function as a two-layerantireflection layer. To this end, the third lens layer 34 is made ofSiO or SiOC, as in the configuration (1) described above.

For example, when the first lens layer 31 is made of SiON, the secondlens layer 33 is made of SiON having the same refractive index as therefractive index of SiON, which forms the first lens layer 31, or SiNhaving a refractive index higher than the refractive index of SiON,which forms the first lens layer 31. The third lens layer 34 is made ofSiOC or SiO having a refractive index lower than the refractive index ofSiON, which forms the first lens layer 31.

When the first lens layer 31 is made of SiN, the second lens layer 33 ismade of SiN having the same refractive index as the refractive index ofSiN, which forms the first lens layer 31. The third lens layer 34 ismade of SiOC or SiO having a refractive index lower than the refractiveindex of SiN, which forms the first lens layer 31.

The refractive index of the material that forms each of the lens layersis determined by a variety of film formation conditions in a P-CVDprocess (plasma CVD, plasma-enhanced chemical vapor deposition), such asthe temperature, the pressure, the type of gas, and the flow rate of thegas. The refractive indices of the first lens layer 31 and the secondlens layer 33 are adjusted to be higher than the refractive index of atypical microlens resin material (ranging from about 1.5 to 1.6) inorder to improve the ability of the microlenses 30 to collect light.

When the second lens layer 33 and the third lens layer 34 function as atwo-layer antireflection layer, the second lens layer 33 is made of ahigh refractive index material, and the third lens layer 34 is made of alow refractive index material. The high refractive index material thatforms the second lens layer 33 is a material having a refractive indexhigher than the refractive index of SiN, such as a zirconium oxide (ZrOhaving a refractive index n of 2.4) or a titanium oxide (TiO having arefractive index n of 2.52). The low refractive index material thatforms the third lens layer 34 is a material having a refractive indexlower than the refractive index of SiOC, such as a magnesium fluoride(MgF₂ having a refractive index n of 1.37). Each of the second lenslayer 33 and the third lens layer 34 is formed to a thickness of 1.0/4λ,where λ represents the wavelength of desired light to be reflected.

<2. Method for Manufacturing Solid-State Imaging Device>

A description will next be made of a method for manufacturing thesolid-state imaging device according to the embodiment described above.In the following description, only the components located above thecolor filters in the solid-state imaging device are shown, and the othercomponents are omitted. The components located below the color filterscan be manufactured by using a method for manufacturing a solid-stateimaging device known in related art. Further, the solid-state imagingdevice described below has a configuration in which no buffer layer isprovided between the second planarization layer and the first lenslayer.

First, the color filters 26 to be formed in correspondence with thepixels of the solid-state imaging device are formed, as shown in FIG.6A. The color filters 26 are formed by using a coloring agent, forexample, a pigment or a photosensitive resin into which a pigment isadded, in a photolithography process. The color filters 26 are made ofRED, GREEN, BLUE, YELLOW, CYAN, and MAGENTA and other color materials.

The second planarization layer 27 is then formed on the color filters26, as shown in FIG. 6B. The second planarization layer 27 is made of amaterial that has thermal fluidity and thermal curability and forms acured layer when a thermal treatment is finished, such as an acrylicresin, a styrene resin, and a styrene-acryl copolymerizing resin. Amethod for forming the second planarization layer 27 will be describedlater in detail.

The first lens layer 31 made, for example, of SiN is then formed on thesecond planarization layer 27, as shown in FIG. 6C. The first lens layer31 is formed to be sufficiently thicker than the first microlenses 32 tobe formed. The first lens layer 31 is formed, for example, in a P-CVDprocess using SiH₄, NH₃, and N₂ as film formation gases. The pressureand other parameters are adjusted as appropriate at a temperature ofabout 200° C. in the P-CVD formation process.

A positive photosensitive resin 41 is formed on the first lens layer 31and patterned in correspondence with the pixels of the solid-stateimaging device 20, as shown in FIG. 6D. Examples of the positivephotosensitive resin 41 include a novolac resin, a styrene-based resin,and a copolymerizing resin formed thereof. The photosensitive resin 41is formed and patterned, for example, by performing the followingprocesses: spin application; pre-baking; i-line light exposure;post-exposure baking, development, and post-baking processing in thisorder. In the post-baking processing, the photosensitive resin 41 havingthe lens shape shown in FIG. 6D is formed.

The positive photosensitive resin 41 described above is then used as amask to transfer the lens shape made of the photosensitive resin 41 tothe first lens layer 31 in an etching process. The first microlenses 32are thus formed, as shown in FIG. 7E. The first lens layer 31 is etched,for example, by using an ICP (inductively coupled plasma) apparatus, aCCP (capacitively coupled plasma) apparatus, a TCP (transformer coupledplasma) apparatus, a magnetron RIE (reactive ion etching) apparatus, anECR (electron cyclotron resonance) apparatus, or any other suitableplasma generating apparatus. The temperature, the pressure, and otherparameters are then adjusted as appropriate, and an etching gasprimarily made of CF₄, C₄F₈, or any other fluoro-carbon-based gas isused.

A plan view of the first lens layer 31 to which the lens shape shown inFIG. 7E has been transferred shows that the inter-pixel gaps w arepresent in at least one of the directions W1 and W2 associated with thefirst microlenses 32, as shown in FIG. 3 described above. The etchingcan be finished in a short period by forming the first lens layer 31 ina condition that allows the inter-pixel gaps w to be present asdescribed above. Since the dry etching period can be shortened, anincrease in dark current in the solid-state imaging device 20 due toplasma damage can be suppressed.

The second lens layer 33 is then formed on the first lens layer 31, asshown in FIG. 7F. The second lens layer 33 formed on the firstmicrolenses 32 follows the shape of the lens surfaces of the firstmicrolenses 32. Further, the second lens layer 33 formed on theinter-pixel gaps w is thicker than the second lens layer 33 formed onthe first microlenses 32.

The second lens layer 33 is made, for example, of SiN and formed in aP-CVD process by using SiH₄, NH₃, and N₂ as gases for forming an SiNfilm. The pressure and other parameters are adjusted as appropriate at atemperature of about 200° C. in the P-CVD formation process.

The thus formed second lens layer 33 is then etched to form the secondlens layer 33 having the configuration shown in FIG. 7G or 7H.

FIG. 7G corresponds to the configuration of the microlenses 30A shown inFIG. 4A described above, in which the second lens layer 33 is formedover the entire surface of the first lens layer 31. The second lenslayer 33 is thinnest at the central portion of each of the firstmicrolenses 32, and the layer thickness gradually increases from thecentral portion toward the periphery.

The second lens layer 33 fills the inter-pixel gaps w between the firstmicrolenses 32. The microlenses 30 are thus formed.

FIG. 7H corresponds to the configuration of the microlenses 30B shown inFIG. 4B described above, in which the second lens layer 33 is formedover the entire surface of the first lens layer 31 except the centralportion of each of the first microlenses 32. The etching of the secondlens layer 33 is terminated when the layer thickness at the centralportion of each of the first microlenses 32 becomes zero.

As shown in FIGS. 7G and 7H, the second lens layer 33 formed at theperiphery of each of the first microlenses 32 fills the inter-pixel gapw, and the resultant microlenses 30 have no inter-pixel gap w.

Although not shown, the configuration of the microlenses 30C shown inFIG. 4C described above and the configuration of the microlenses 30Dshown in FIG. 4D described above can also be achieved by changing theetching conditions as appropriate.

The solid-state imaging device 20 including the microlenses 30 shown inFIG. 2 described above can be manufactured by carrying out the stepsdescribed above.

According to the manufacturing method described above, the firstmicrolenses 32 are formed with the inter-pixel gaps w present in atleast one of the direction parallel to horizontally or verticallyadjacent pixels (W1) and the direction parallel to diagonally adjacentpixels (W2). In the condition that allows the inter-pixel gaps w to beformed, the dry etching period can be shortened, whereby an increase indark current in the solid-state imaging device 20 is suppressed.Further, the second lens layer 33 fills the inter-pixel gaps w and theresultant microlenses 30 have no inter-pixel gap w, whereby the area ofeach of the microlenses 30 in a plan view is enlarged and the ability ofthe microlens 30 to collect light is improved accordingly. As a result,the sensitivity and shading characteristics of the solid-state imagingdevice 20 can be improved.

[Method for Forming Second Planarization Layer]

A description will be made of a method for forming the secondplanarization layer 27 formed on the color filters 26 in the method formanufacturing the solid-state imaging device described above.

To increase the sensitivity of the solid-state imaging device 20, it ispreferable to shorten the distance between the microlenses 30 and thephotodiodes 22. To this end, each of the layers on the semiconductorbase substrate 21 shown in FIG. 2 described above is desirably formed tobe thin. The second planarization layer 27, when formed, is alsodesirably formed to be thin.

To reduce the thickness of the second planarization layer 27, the secondplanarization layer 27 is made of a material that has thermal fluidityand thermal curability and forms a cured layer when a thermal treatmentis finished, such as an acrylic resin, a styrene resin, and astyrene-acryl copolymerizing resin.

FIGS. 8A to 8C show steps of forming the second planarization layer 27with a resin having the characteristics described above.

First, a resin is applied onto the color filters 26 in a spinapplication process to form a second planarization layer 27A, as shownin FIG. 8A. The second planarization layer 27A immediately after theapplication has irregularities in the surface because affected by theprotrusions and recesses in the surfaces of the color filters 26 havingdifferent thicknesses for different colors.

The second planarization layer 27A in the state shown in FIG. 8A is thenheat treated. Since the resin that forms the second planarization layer27A has thermal fluidity and thermal curability as described above, theheat treatment increases the fluidity of the second planarization layer27A, which therefore moves in such a way that the surface thereof isplanarized. As a result, a second planarization layer 27B havingrecesses shallower than those before the protrusions and recessesundergo the heat treatment is formed, as shown in FIG. 8B.

FIG. 8C shows the second planarization layer 27 cured by the heattreatment described above. The heat treatment also increases thermalcurability of the second planarization layer 27 as well as the thermalfluidity thereof. The second planarization layer 27 further experiencesthermal shrinkage in the heat treatment. The volume of the secondplanarization layer 27 therefore decreases from that before cure to thatafter cure because the second planarization layer 27 shrinks when itthermally cures. As a result, the cured second planarization layer 27 isthinner than the second planarization layer 27 before cure. The increasein fluidity resulting from the heat treatment further planarizes thesurface of the second planarization layer 27.

Therefore, forming the second planarization layer 27 with a materialthat has thermal fluidity and thermal curability and forms a cured layerwhen a thermal treatment is finished reduces the amount ofirregularities produced immediately after the spin application, wherebya substantially flat, thin, thermally cured layer can be formed.

Further, the second planarization layer 27, which is made, for example,of an acrylic resin, a styrene resin, or a styrene-acryl copolymerizingresin, which experiences increases in thermal fluidity and thermallycurability at the same time, can be formed to be further thinner byreducing the molecular weight of the resin to increase the amount ofthermal shrinkage. The thus formed thin second planarization layer 27reduces the distance between the photodiodes 22 and the microlenses 30,whereby the sensitivity characteristics of the solid-state imagingdevice 20 are improved.

[Etching Transfer to First Lens Layer]

A description will be made of the step of transferring the lens shapemade of the photosensitive resin 41 to the first lens layer 31 shown inFIGS. 6D and 7E in the method for manufacturing the solid-state imagingdevice described above.

The etching transfer described above is preferably performed in acondition that allows the material of the first lens layer 31 to beetched at the same speed as the photosensitive resin 41 formed on thefirst lens layer 31 is etched.

FIG. 9 shows not only a state in which the photosensitive resin 41having the microlens shape is formed on the first lens layer 31 but alsothe shape of the first microlenses 32 transferred to the first lenslayer 31 in the etching transfer process. In FIG. 9, h1 represents theheight (thickness) of the central portion of the lens shape made of thephotosensitive resin 41 and w3 represents the inter-pixel gap of thelens shape. Further, h2 represents the height (thickness) of the centralportion of each of the first microlenses 32, and w4 and w5 represent theinter-pixel gap between the first microlenses 32.

When the first lens layer 31 is made of SiN, the etching is performed ina condition that allows (speed at which SiN is etched):(speed at whichphotosensitive resin 41 is etched) to be 1:1. The shape of the firstmicrolenses 32 formed in this process is indicated by the broken line32A in FIG. 9. When the ratio of the etching speed between the firstlens layer 31 and the photosensitive resin 41 is 1:1, the shape of thefirst microlenses 32 after the etching is substantially the same as theshape of the photosensitive resin 41. As a result, the central portionof each of the first microlenses 32 is formed to a height h2 equal tothe height h1 of the photosensitive resin 41. The first microlenses 32are also formed to have an inter-pixel gap w4 equal to the inter-pixelgap w3 of the photosensitive resin 41.

Further, when a gas having strong etching deposition, such as C₄F₈, isused in the etching transfer process described above, the speed at whichthe first lens layer 31 is etched decreases because the ratio of theetching speed between the first lens layer 31 and the photosensitiveresin 41 is shifted from 1:1. The solid line in FIG. 9 represents theshape of the first microlenses 32 formed under the condition describedabove.

When the first microlenses 32 are etched to the height h2 under thecondition described above, the inter-pixel gap w5 is smaller than thatachieved when the etching ratio is 1:1. That is, the inter-pixel gap w5between the first microlenses 32 and the inter-pixel gap w3 associatedwith the photosensitive resin 41 satisfy the relationship of w3>w5.

Since the inter-pixel gap w5 between the first microlenses 32 is small,the second lens layer formed on the first lens layer 31 can be thinnerbut can still fill the inter-pixel gap w5.

Further, when the speed at which the first lens layer 31 is etchedrelatively decreases, the etching process period increases, which raisesa concern about an increase in plasma damage to the solid-state imagingdevice 20. To reduced the plasma damage, it is necessary to increase theetching speed. In this case, the flow rate of the C₄F₈ gas is reduced ora variety of etching conditions are so adjusted that the speed at whichthe first lens layer 31 is etched is greater than the speed at which thephotosensitive resin 41 is etched with a gas type of C₄F₈ still used. Asa result, degradation in dark current characteristics due to plasmadamage to the solid-state imaging device 20 and a decrease inproductivity thereof can be suppressed.

A description will further be made of a case where the etching isperformed in a condition that allows the speed at which the first lenslayer 31 is etched to be relatively so increased that the relationshipamong the widths w3, w4, and w5 is changed from w3>w4 (w5) to w3<w4(w5). When the etching is performed in the condition that allows the gapassociated with the first lens layer 31 increases, the etched first lenslayer 31 may have an aspheric shape shown in FIG. 10 in some casesbecause the first lens layer 31 (for example, SiN) is etched at a highspeed. In this case, each of the first microlenses 32 has a shapesimilar to a cone having a round vertex, and the inter-pixel gap wwidens, resulting in an imperfect arcuate curved portion y.

As described above, even when each of the first microlenses 32 is formedto have a conical shape, the second lens layer 33 is formed on the firstlens layer 31 in the microlenses according to the present embodiment. Asa result, even when each of the first microlenses 32 has an asphericshape, the surface of each of the microlenses 30 can be so corrected byoptimizing the lens shape of the second lens layer 33 that the asphericshape approaches a spherical shape as shown in FIG. 10.

Therefore, even when there is a decrease in light collecting abilityresulting from the shape of the first microlenses 32, the ability of themicrolenses 30 to collect light can be improved by forming the secondlens layer 33.

[Conditions Under which Second Lens Layer is Formed]

A description will be made of the relationship between the conditionsunder which the second lens layer 33 is formed and the curvature of thelens shape of the surface of the second lens layer 33 to be formed.

FIGS. 11A and 11B show a method for forming the second lens layer 33with SiN or SiON, and a variety of specific conditions under which themean free path is adjusted to adjust the curvature of the microlenses 30in a P-CVD process are shown below.

(1) Silicon Nitride (SiN)

Gas: SiH₄, NH₃, N₂

Temperature: about 200° C.

(2) Silicon oxynitride (SiON)

Gas: SiH₄, NH₃, N₂O, N₂

Temperature: about 200° C.

The pressure at the time of film formation is adjusted to be a valuebetween about 2 mTorr to 10 Torr. The mean free path increases as thepressure decreases, whereas decreasing as the pressure increases. Thatis, the mean free path is long when the pressure is 2 mTorr, whereasbeing short when the pressure is 10 Torr.

FIG. 11A shows the configuration in a case where the second lens layer33 is formed in a condition where the pressure at the time of filmformation is low and hence the mean free path is long. On the otherhand, FIG. 11B shows the configuration in a case where the second lenslayer 33 is formed in a condition where the pressure at the time of filmformation is high and hence the mean free path is short.

The curvature of the surface of the second lens layer 33 decreases whenthe mean free path is long, as shown in FIG. 11A. On the other hand, thecurvature of the surface of the second lens layer 33 increases when themean free path is short, as shown in FIG. 11B.

When the second lens layer 33 at a central portion of each of the firstmicrolenses 32 has the same thickness Tt in both cases, the second lenslayer 33 at the periphery of the first microlens 32 therefore has alarge thickness Tb in FIG. 11A, in which the mean free path is long. Onthe other hand, the second lens layer 33 at the periphery of the firstmicrolens 32 has a small thickness Tb in FIG. 11B, in which the meanfree path is short.

The ratio between the thicknesses of the second lens layer 33 at thecentral portion and the periphery (Tt/Tb) is greater when the pressureat the time of film formation is high than when the pressure at the timeof film formation is low.

As described above, a desired shape can be formed, for example, themicrolenses 30 can be formed in desired positions in the solid-stateimaging device 20 and the microlenses 30 can have desired curvature, byadjusting the conditions under which the second lens layer 33 is formed.

Further, even when the first lens layer 31 is formed to have an asphericshape as shown in FIG. 10 described above, the aspheric shape can becorrected to a nearly spherical shape by adjusting the conditions underwhich the second lens layer 33 is formed as described above.

Although not described in the manufacturing method described above, thebuffer layer 28 shown in FIG. 2 may be formed on the secondplanarization layer 27. In this case, the buffer layer 28 is made, forexample, of SiO or SiON in a P-CVD process.

When the buffer layer 28 is made of SiO, SiH₄, N₂O, and other gases areused as the film formation gases. When the buffer layer 28 is made ofSiON, SiH₄, NH₃, N₂O, N₂, and other gases are used as the film formationgases. The pressure and other parameters are adjusted as appropriate ata temperature of about 200° C. The temperature is determined also inconsideration of color bleaching of the color filters 26 and the heatresistance of the organic material that forms the first planarizationlayer 25 and other layers.

The above embodiment has been described with reference to the case wherethe microlenses are used with a rear-illumination solid-state imagingdevice, but the microlenses described above can be used with afront-illumination CMOS solid-state imaging device and CCD solid-stateimaging device.

<3. Electronic Apparatus>

A description will next be made of an embodiment of an electronicapparatus including the solid-state imaging device described above.

The solid-state imaging device described above can be used in a camerasystem, such as a digital camera and a video camcorder; a mobile phonehaving an imaging capability; and other electronic apparatus having animaging capability. FIG. 12 shows a schematic configuration of anapparatus in which the solid-state imaging device is used with a cameracapable of capturing a still image or video images as an example of anelectronic apparatus.

A camera 50 in this example includes a solid-state imaging device 51, anoptical system 52 that guides incident light to a light receiving sensorportion of the solid-state imaging device 51, a shutter 53 providedbetween the solid-state imaging device 51 and the optical system 52, anda drive circuit 54 that drives the solid-state imaging device 51. Thecamera 50 further includes a signal processing circuit 55 that processesan output signal from the solid-state imaging device 51.

The solid-state imaging device 51 can be either of the solid-stateimaging devices of the embodiment described above and a secondembodiment. The optical system (optical lens) 52 focuses image light(incident light) from a subject on an imaging surface (not shown) of thesolid-state imaging device 51. Signal charge is thus accumulated for afixed period in the solid-state imaging device 51. The optical system 52may be formed of an optical lens group including a plurality of opticallenses. The shutter 53 controls a period during which the solid-stateimaging device 51 is illuminated with the incident light and a periodduring which the incident light to the solid-state imaging device 51 isblocked.

The drive circuit 54 supplies drive signals to the solid-state imagingdevice 51 and the shutter 53. Using the supplied drive signals, thedrive circuit 54 controls signal output operation of the solid-stateimaging device 51 to the signal processing circuit 55 and shutteringoperation of the shutter 53. That is, in this example, one of the drivesignals (timing signal) supplied from the drive circuit 54 allows thesignal from the solid-state imaging device 51 to be transferred to thesignal processing circuit 55.

The signal processing circuit 55 performs a variety of types of signalprocessing on the signal transferred from the solid-state imaging device51. The signal having undergone the variety of signal processing (videosignal) is stored in a memory of any other storage medium (not shown) oroutputted to a monitor (not shown).

The above description has been made with reference to the case where thesolid-state imaging device according to each of the above embodiments isused as an image sensor having unit pixels that are arranged in a matrixand detect signal charge according to the amount of visible light as aphysical quantity. The solid-state imaging device described above canalso be used as the entire range of column-type solid-state imagingdevices having a column circuit provided for each pixel column in apixel array section.

Further, the solid-state imaging device described above is notnecessarily used as a solid-state imaging device that detects thedistribution of incident visible light to capture an image but can beused as a solid-state imaging device that captures an image of thedistribution of the amount of incident infrared light, X rays,particles, or any other substance. Moreover, the solid-state imagingdevice described above can be used as the entire range of solid-stateimaging devices in a broad sense that detect the distribution ofpressure, static capacitance, or any other physical quantity to capturean image (physical quantity distribution detection apparatus), such as afingerprint detection sensor.

Further, the solid-state imaging device described above does notnecessarily sequentially scan unit pixels in a pixel array section on arow basis to read a pixel signal from each of the unit pixels. Forexample, the solid-state imaging device described above can be used asan X-Y addressing solid-state imaging device that selects an arbitrarypixel on a pixel basis and reads a signal from the selected pixel on apixel basis.

Moreover, the solid-state imaging device may be provided in the form ofsingle chip or in the form of module having an imaging capability inwhich an imaging section and a signal processing section or an opticalsystem are packaged together.

The technology of the present disclosure may also be implemented as thefollowing configurations.

(1) A solid-state imaging device including a first lens layer and asecond lens layer, wherein the second lens layer is formed at least at aperiphery of each first microlens formed based on the first lens layer,and the second lens layer present at a central portion of each of thefirst microlenses is thinner than the second lens layer present at theperiphery of the first microlens or no second lens layer is present atthe central portion of each of the first microlenses.

(2) The solid-state imaging device described in (1), wherein the firstlens layer is made of a metal-oxide-containing resin or an inorganicmaterial, and the second lens layer is made of an inorganic material.

(3) The solid-state imaging device described in (1) or (2), wherein thefirst lens layer has an refractive index n1, the second lens layer hasan refractive index n2, and n2≤n1 is satisfied.

(4) The solid-state imaging device described in any of (1) to (3),further including a third lens layer that covers the first lens layerand the second lens layer, and the refractive index of the third lenslayer is lower than the refractive indices of the first lens layer andthe second lens layer.

(5) The solid-state imaging device described in (4), wherein a surfaceof the third lens layer is planarized.

(6) The solid-state imaging device described in any of (1) to (5),wherein the second lens layer formed at the central portion of each ofthe first microlenses has a thickness Tt, the second lens layer formedat the periphery of the first microlens has a thickness Tb, and 0≤Tt<Tbis satisfied.

(7) A method for manufacturing a solid-state imaging device including afirst lens layer and a second lens layer, the method including formingfirst microlenses having an inter-pixel gap therebetween based on thefirst lens layer, and forming the second lens layer at least at aperiphery of each of the first microlenses, wherein in the formation ofthe second lens layer, the second lens layer formed at a central portionof each of the first microlenses is thinner than the second lens layerformed at the periphery of the first microlens or no second lens layeris present at the central portion of each of the first microlenses.

(8) An electronic apparatus including the solid-state imaging devicedescribed in any of (1) to (6) and a signal processing circuit thatprocesses an output signal from the solid-state imaging device.

The present disclosure contains subject matter related to that disclosedin Japanese Priority Patent Application JP 2011-217423 filed in theJapan Patent Office on Sep. 30, 2011, the entire contents of which arehereby incorporated by reference.

It should be understood by those skilled in the art that variousmodifications, combinations, sub-combinations and alterations may occurdepending on design requirements and other factors insofar as they arewithin the scope of the appended claims or the equivalents thereof.

The invention claimed is:
 1. An imaging device comprising a micro lenscomprising: a first lens layer comprising a plurality of first microlenses with inter pixel gaps therebetween; and a second lens layer abovethe first lens layer; wherein a thickness of the second lens layer at acentral portion of one of the first micro lenses in the plurality offirst micro lenses is less than a thickness of the second lens layer ata periphery of the one of the first micro lenses; wherein the secondlens layer is disposed in a continuous layer at least above the interpixel gap; and wherein the second lens layer has a thickness greaterthan zero throughout the inter pixel gap.
 2. The imaging deviceaccording to claim 1, wherein the first lens layer includes at least onematerial selected from among the group consisting of silicon, an oxide,a nitride and any combination thereof.
 3. The imaging device accordingto claim 1, wherein the first lens layer is selected from among thegroup consisting of SiON, SiN and any combination thereof.
 4. Theimaging device according to claim 1, wherein the second lens layerincludes at least one material selected from among the group consistingof silicon, an oxide, a nitride, carbon and any combination thereof. 5.The imaging device according to claim 1, wherein the second lens layeris selected from the among the group consisting of SiON, SiN, SiO, SiOCand any combination thereof.
 6. The imaging device according to claim 1further comprising a pixel section and a peripheral circuit section. 7.The imaging device according to claim 6, wherein the pixel sectionincludes a transfer transistor, a rest transistor and an amplificationtransistor.
 8. The imaging device according to claim 7, wherein thepixel section further includes a selection transistor.
 9. The imagingdevice according to claim 6, wherein the peripheral circuit sectionincludes a vertical drive circuit, one or more column processingcircuits, a horizontal drive circuit, an output circuit, and a controlcircuit.
 10. The imaging device according to claim 9, wherein the one ormore column processing circuits are configured to perform correlateddouble sampling (CDS).
 11. The imaging device according to claim 1further comprising a substrate that includes one or more photodiodes,wherein the first microlens, the second microlens and the inter-pixelgap are disposed above the substrate.
 12. The imaging device accordingto claim 11, further comprising an insulating layer formed of a singlelayer or multiple layers, wherein the insulating layer is disposed abovethe substrate and wherein the insulating layer is disposed below thefirst microlens, the second microlens and the inter-pixel gap.
 13. Theimaging device according to claim 12, wherein the insulating layercomprises a single-layer structure of SiO or a multiple-layer structureof HfO₂ and SiO.
 14. The imaging device according to claim 13 furthercomprising an inter-pixel blocking layer formed from at least onematerial selected from among the group consisting of W, Al, Cu, a carbonblack, a titanium black, a metal black, or an organic material.
 15. Theimaging device according to claim 14 further comprising a firstplanarization layer formed on the insulating layer.
 16. The imagingdevice according to claim 15, wherein the first planarization layerincludes an acrylic resin.
 17. The imaging device according to claim 16further comprising one or more color filters formed on the firstplanarization layer.
 18. The imaging device according to claim 17further comprising a second planarization layer formed on the one ormore color filters.
 19. The imaging device according to claim 18,wherein the second planarization layer includes an acrylic resin, astyrene resin, or a styrene-acryl copolymerizing resin.
 20. The imagingdevice according to claim 19 further comprising a buffer layer formed onthe second planarization layer and including SiO or SiON.